1. Field of the Invention
The invention relates to a high-pass filter, and more particularly, to an on-chip high-pass filter with a large time constant.
2. Description of the Prior Art
In order to remove the DC component of a signal or to eliminate the DC offset, many circuits call for the use of a high-pass filter. A DC voltage blocking capacitor, DC voltage level shifter, and DC servo loop are all the example circuits implemented as high-pass filters.
In a device using two separate supply voltages, it may not be possible to directly connect signals from one power supply domain to another due to different DC voltage levels. In order to connect signals between the two domains, a DC voltage level shifter can be used to shift the voltage from the first domain to that of the second domain. However, due to improvements in the IC and device fabrication process, the on-chip working voltage has been greatly reduced and is actually too low for a DC voltage level shifter to meet its original performance requirements. In this situation, a high-pass filter can be used to block the DC voltage but still pass the desired signal. If the corner frequency of the high-pass filter is low enough, there will be no adverse effect on the system by using a high-pass filter.
DC offset voltage is always a concern when implementing zero intermediate frequency (IF) receivers, mixers, and low pass filters. If not removed, this unwanted DC offset voltage could saturate sensitive devices, such as ADC converters, causing them to malfunction. A normal solution to this problem is to add a DC servo loop. A DC servo circuit uses feedback to hold the output DC voltage level constant, however, the smaller the feedback window is (meaning a very low corner frequency), the longer the settling time will be. When the corner frequency is very low, not only are the resistance of the resistor and the capacitance of the capacitor of the required filter very large but also the settling time is often too long. By using a simple high-pass filter, the same function as a DC servo circuit can be achieved without the added problem of the closed loop settling time being too long.
FIG. 1 shows a conventional high-pass filter 10. A capacitor 12 is connected between an input port 14 and an output port 16 and a resistor 18 is connected between the output port 16 and ground. When using the standard resistor/capacitor (RC) high-pass filter 10 shown in FIG. 1, a resistor with large resistance 18 and a capacitor with large capacitance 12 must be used in order to produce the desired low corner frequency. If the corner frequency is specified to be 100 Hz (or lower), the resistance of the resistor 18 must be in the order of Mohms and the capacitance of the capacitor 12 will approach the hundred pF range. For example, if the desired corner frequency (Fc) is 100 Hz, using the formula Fc=1/(2Ï*C), the capacitance of the capacitor 12 is 50 pF and the resistance of the resistor 18 must be 33 Mohm. These large-magnitude resistors and capacitors are very difficult to implement on an IC without using extremely large amounts of IC space. The space required by the high-pass filter 10 could actually be several times the space of the rest of the design.